Summary
A Stored Cross-site Scripting (XSS) vulnerability exists in the comment and issue description functionality. The application's HTML sanitizer explicitly allows data: URI schemes, enabling authenticated users to inject arbitrary JavaScript execution via malicious links.
Details
The vulnerability is located in internal/markup/sanitizer.go. The application uses the bluemonday HTML sanitizer but explicitly weakens the security policy by allowing the data URL scheme:
// internal/markup/sanitizer.go
func NewSanitizer() {
sanitizer.init.Do(func() {
// ...
// Data URLs
sanitizer.policy.AllowURLSchemes("data")
// ...
})
}While the Markdown renderer rewrites relative links (mitigating standard Markdown [link](data:...) attacks), Gogs supports Raw HTML input. Raw HTML anchor tags bypass the Markdown parser's link rewriting and are processed directly by the sanitizer. Since the sanitizer is configured to allow data: URIs, payloads like <a href="data:text/html..."> are rendered as-is.
PoC
- Create a file named
exploit.md in a repository.
- Add the following content (Raw HTML):
<a href="data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4=">Click me for XSS</a>
- Commit and push the file.
- Navigate to the file in the Gogs web interface.
- Click the "Click me for XSS" link.
- Result: An alert box with "XSS" appears, executing the JavaScript payload.
Impact
This is a Stored XSS vulnerability. Any user who views the malicious comment and clicks the link will execute the attacker-supplied JavaScript in their browser context. This allows attackers to:
- Steal authentication cookies and session tokens.
- Perform arbitrary actions on behalf of the victim (e.g., modifying repositories, adding collaborators).
- Redirect users to malicious sites.
References
dxlerYT Reporter
Summary
A Stored Cross-site Scripting (XSS) vulnerability exists in the comment and issue description functionality. The application's HTML sanitizer explicitly allows
data:URI schemes, enabling authenticated users to inject arbitrary JavaScript execution via malicious links.Details
The vulnerability is located in
internal/markup/sanitizer.go. The application uses thebluemondayHTML sanitizer but explicitly weakens the security policy by allowing thedataURL scheme:While the Markdown renderer rewrites relative links (mitigating standard Markdown
[link](data:...)attacks), Gogs supports Raw HTML input. Raw HTML anchor tags bypass the Markdown parser's link rewriting and are processed directly by the sanitizer. Since the sanitizer is configured to allowdata:URIs, payloads like<a href="data:text/html...">are rendered as-is.PoC
exploit.mdin a repository.Impact
This is a Stored XSS vulnerability. Any user who views the malicious comment and clicks the link will execute the attacker-supplied JavaScript in their browser context. This allows attackers to:
References